1. Field of the Invention
The present invention relates to a surface acoustic wave device which operates in a high-frequency range including several hundreds of MHz to GHz and more particularly to such a surface acoustic wave device which comprises diamond or a diamond-like carbon film, and to a piezoelectric body, and also relates to a method for manufacturing the same.
2. Related Background Art
A surface acoustic wave device is an electromechanical conversion device utilizing a surface wave propagating on the surface of an elastic body and has the basic structure shown in FIG. 1. In a surface acoustic wave device 40, the piezoelectric phenomenon of a piezoelectric body 44 is used in exciting a surface acoustic wave. When an electrical signal is applied to one interdigital transducer (comb-like electrode) 43 formed on the piezoelectric body 44, the piezoelectric body 44 is stressed, and this stress becomes a surface acoustic wave. Then, the surface acoustic wave propagates on the piezoelectric body 44 and is extracted as an electrical signal at another interdigital transducer 43'. The frequency characteristics of the surface acoustic wave device include a band passing characteristic with a center frequency f.sub.0 defined by f.sub.0 =V/.lambda..sub.0, where V is a propagation velocity of the surface acoustic wave and .lambda..sub.0 is a space between each of the electrodes of the interdigital transducer.
The surface acoustic wave device requires a small number of parts and can be miniaturized. In addition, signals can be easily coupled into and out of a surface acoustic wave propagation path. This device can be used in a variety of applications such as a filter, a delay line, an oscillator, a resonator, a convolver, or a correlator. In particular, the surface acoustic wave device has been used as an IF television filter. Such surface acoustic wave devices have also been tested as filters for VTRs and various communication apparatus such as car telephones, cellular phones and so on.
A typical conventional surface acoustic wave device has a structure in which interdigital transducers are formed on a crystalline piezoelectric body such as LiNbO.sub.3 or LiTaO.sub.3. Another surface acoustic wave device having a piezoelectric thin film of ZnO or the like sputtered on a base substrate of glass or the like has also been used.
It is, however, difficult to manufacture a device operating in a high-frequency (GHz band) range using the conventional device structure described above. A device in which interdigital transducers are simply formed on a single crystalline piezoelectric body without any other consideration cannot have a high center frequency in excess of 1 GHz because its surface acoustic wave propagation velocity V is too low.
As indicated by the above equation, in order to achieve a surface acoustic wave device having band passing characteristics with a higher center frequency, the space .lambda..sub.0 between each of the electrodes has to be smaller or the surface acoustic wave propagation velocity V has to be increased.
Decreasing the space .lambda..sub.0 between each of the electrodes to increase the center frequency is limited by the capabilities of microlithography techniques such as photolithography.
For this reason, various techniques for increasing the propagation velocity V of the surface acoustic wave have been examined.
A device in which a sapphire layer having a larger propagation velocity than that of the piezoelectric body for the surface acoustic wave is placed between a base substrate and a piezoelectric layer is disclosed in Japanese Patent Laid-Open No. 50-154088(1975) corresponding to Japanese Koukoku (Opposition) Publication No. 54-38874(38874/1979). Further, a device in which a piezoelectric layer is deposited on a diamond layer in order to increase the propagation velocity of the surface acoustic wave is disclosed in Japanese Patent Laid-Open No. 64-62911(62911/1989) by Imai et al. and Japanese Patent Laid-Open No. 3-198412(198412/1991) by Nakahata et al. Present FIGS. 2 to 5 show the devices disclosed in these references.
In a device shown in FIG. 2, a piezoelectric layer 54 is formed on a diamond layer 52 and between these layer, interdigital transducers 53 are formed. In a device shown in FIG. 3, short circuiting electrodes 56 are placed on the piezoelectric layer 54 of the device shown in FIG. 2. In a device shown in FIG. 4, a piezoelectric layer 54 is formed on a diamond layer 52, and interdigital transducers 53 are placed on the piezoelectric layer 54. The device shown in FIG. 5 comprises the short circuiting electrodes 56 placed between the piezoelectric layer 54 and the diamond layer 52 of the device shown in FIG. 4.
As described above, it has been known that placing the short circuiting electrodes 56 between the piezoelectric layer 54 and the diamond layer 52 in the surface acoustic wave device having such a structure shown in FIG. 4 can achieve a higher electromechanical coupling coefficient.
However, it has been found that the yield is reduced if the surface acoustic wave device with such a structure is manufactured.